sessions: export as markdown — the sidebar right-click, the route, the practice join (release polish 3)

Owner round (2026-10-06): markdown export of sessions, actioned from
the sidebar context menu — "Export as Markdown…" sits with the
management actions, above Delete (the destructive action stays last).

- GET /api/chats/:id/markdown (404 unknown): the whole session as one
  attachment — text/markdown; charset=utf-8, filename from the slugified
  title ("relational-databases-joins-normal-forms.md", non-ascii titles
  fall back to "session.md"). Both session kinds export.
- The client is a same-origin anchor download; the route's
  Content-Disposition names the file.
- sessionMarkdown's user-facing debut — minted ANCHORLESS (message-id
  anchors are extraction provenance, not learner-facing). The engine
  joins the core barrel (pure, browser-safe) for its first consumer.

The practice join (owner-approved mid-round): practice tool results are
bare acks — statement, outcome, verdict, and the model solution all
live in the assistant's call arguments. The engine now folds them the
same way the view seam folds cards: Problem blocks render the pose's
statement (solution never leaks at pose time); Concluded blocks render
the outcome as prose ("Solved with hints.") + verdict, then the
pose-committed model solution as a blockquoted reveal via
problemInputForConclude (clean-poses-only, its refusal law). A
dismissed problem concludes as "The learner skipped this problem." —
no reveal. Errored poses render nothing (the failed-tool law).

1427 tests (route pins: headers, slug, anchorless body, 404; engine
pins: pose-time statement, the reveal, the dismissal refusal, the
errored-pose silence); tsc, lint, build green.

components/sidebar.tsx

@@ -864,6 +864,28 @@ function SessionsSidebar() {
 						>
 							Rename
 						</button>
+						{/* Export as Markdown (owner, release polish 2026-10-06): the
+					    session-markdown engine's user-facing debut — same-origin
+						    anchor download; the route's Content-Disposition names the
+						    file. Sits with the management actions, above Delete (the
+						    destructive action stays last — the danger-fence law). */}
+						<button
+							type="button"
+							data-testid="ctx-export-md"
+							onClick={() => {
+								const chatId = ctxChatId;
+								setCtxChatId(null);
+								const a = document.createElement("a");
+								a.href = `/api/chats/${chatId}/markdown`;
+								a.download = ""; // the route's filename wins
+								document.body.appendChild(a);
+								a.click();
+								a.remove();
+							}}
+							className="flex items-center rounded-lg px-2.5 py-1.5 text-left text-foreground transition-colors hover:bg-accent"
+						>
+							Export as Markdown…
+						</button>
 						<button
 							type="button"
 							data-testid="ctx-delete"